Apparatus for switching windings of ac three-phase motor

ABSTRACT

It is an object to provide a winding switching device of a three-phase AC motor having a small size at a low cost in which a time required for switching a winding is shortened and the number of semiconductor switch units is decreased as much as possible.  
     In a winding switching device of a three-phase AC motor including an AC motor in which a winding having each phase is formed by a plurality of windings and a connecting terminal connecting the windings to each other and both terminals of the winding having the phase are provided on an outside of a motor, winding switching means for properly switching the connecting terminal, and a variable frequency power source for supplying a variable voltage having a variable frequency to the AC motor, the winding switching means is constituted by a plurality of three-phase rectifying means connecting one of ends of the winding having the phase to the variable frequency power source and connecting the other end and the connecting terminal to an input terminal on an AC side of the three-phase rectifying means for each phase, and a semiconductor switch provided to open and close both ends on a DC output side of the three-phase rectifying means.

TECHNICAL FIELD

[0001] The present invention relates to a winding switching device of athree-phase AC motor for enlarging a speed control range by switchingthe winding of the three-phase AC motor, and is intended for a widerange of industrial field including the driving operation of a vehicle,the driving operation of the main shaft of a machine tool, the traversemotion and running of a crane, a winding machine and a servo device.

BACKGROUND ART

[0002] In the driving devices of the main shaft of a machine tool and avehicle which are to be driven by an AC variable frequency power source,a winding switching method has been employed as means capable ofobtaining a sufficiently great torque in a low speed region and carryingout an operation in a high speed region.

[0003] As an example, a star-delta switching method shown in FIG. 6 haswidely been used practically for the driving operation of the main shaftof a machine tool. In FIG. 6, 22 denotes a power source, 16 to 21 denotea diode constituting a three-phase full-wave rectifying bridge, and 15denotes a smoothing capacitor. 14 denotes a converter section forconverting the AC power source 22 into a DC power source. Terminals TPand TN are DC output terminals of the converter section 14, and serve asinputs of the inverter section 1. 2 denotes an AC motor, T1 to T6 denotea terminal to be used in switching, and 3 and 4 denote a switch such asan electromagnetic contactor. When the switch 4 is opened and the switch3 is closed, a star connection is obtained. When the switch 4 is closedand the switch 3 is opened, a delta connection is obtained. Ni denotes aneutral point. In the low speed region, a star (Y) connection isselected to apply a sufficiently high voltage so that a great torque canbe obtained for the same current. Since the impedance of the motor isincreased in proportion to a frequency, the current flow might beimpeded in the high speed region in which the frequency is increased. Byselecting a delta (Δ) connection having a low impedance, therefore, thecurrent flow can be conducted easily.

[0004] In FIG. 7, two sets of star windings are switched in series andparallel. A switch 5 is closed to connect the windings in series at alow speed, and switches 6 and 7 are closed to connect the windings inparallel at a high speed. Consequently, the same advantage as that ofFIG. 6 can be obtained. Furthermore, FIG. 8 is obtained by simplifyingthe circuit in FIG. 7. When a switch 8 is closed, all windings areutilized equivalently to a series connection. When a switch 9 is closed,a part of the windings is used so that a characteristic corresponding toa parallel connection in FIG. 7 is obtained. In this case, the residualwindings are not used but in an idle state. For this reason, a currentdensity is increased to be a double of that in FIG. 7. However, thenumber of winds for creating a magnetic flux is equal. Consequently, aninduced voltage and a torque characteristic are basically equivalent tothose in the parallel connection.

[0005] While switching in two stages is carried out in all the examplesdescribed above, a method of carrying out switching in three stages, andfurthermore, a more precise control has been disclosed in U.S. Pat. No.3,037,471.

[0006] In all the examples described above, it is assumed that switchingis carried out by a switch having a mechanical contact. There has beenmade a proposal for reducing a dead time for switching with theoperating time of the switch. FIG. 9 has been disclosed in JP-B-7-99959by the applicant, in which two sets of inverters are combined to switcha star connection and a delta connection without contact by a change ina method of controlling each inverter. FIG. 10 has been published inIEEE Transactions on Industry Applications, Vol. 32d No. 4, July/August,1996, pp. 938-944. Two sets of windings having different specificationswhich are provided in the same motor are driven by two inverters and acombination of respective current vectors is changed, thereby switchingdouble-pole and four-pole characteristics.

[0007] Moreover, U.S. Pat. No. 2,742,800 has disclosed a method ofapplying circuits connecting, as a switching unit, a semiconductorcontrol unit and a diode for blocking a reverse voltage in series whichare connected in antiparallel based on the circuit in FIG. 8.

[0008] In the methods shown in FIGS. 6, 7 and 8 and the technology inthe U.S. Pat. No. 3,037,471, the switching is carried out with a switchhaving a contact. Accordingly, it is necessary to take a time for such amechanism operation as to turn on and off the contact. Moreover, it isdesirable that a current should be once blocked on an inverter side tocarry out so-called non-current switching in consideration of thelifetime of the contact. When these operating times are summed, the deadtime that cannot be disregarded (usually, several tens milliseconds) isgenerated. For example, the dead time influences the quality of an endproduct in a driving device for the main shaft of a machine tool, andfurthermore, influences a feeling of riding in a driving device for avehicle. A limited contact lifetime itself is also a disadvantage whichcannot be permitted.

[0009] In the methods of FIGS. 8 and 9 and the U.S. Pat. No. 2,742,800,the switching is carried out by opening and closing through thesemiconductor unit or a change in a control mode. Consequently, theproblem of the operating time can be improved. Since the number ofactive semiconductor units to be required is large one, a cost might bea factor in impediment to practical use.

[0010] In the methods of FIG. 8 and the U.S. Pat. No. 2,742,800,furthermore, a voltage induced into the residual winding portions isadded to a supply voltage and a high voltage is applied to an unusedterminal when a power is supplied to the neutral point of the winding.Therefore, it is necessary to intensify an insulation.

DISCLOSURE OF THE INVENTION

[0011] The invention has been made in consideration of the problems andhas an object to provide a winding switching device of a three-phase ACmotor implementing the following (1) to (3).

[0012] (1) A time required for switching a winding is shortened.

[0013] (2) The number of semiconductor switch units for switching awinding is reduced as much as possible to reduce a size and a costwithout using a switch having a mechanical movable section.

[0014] (3) Also in the case in which an intermediate point power of awinding is to be supplied, a voltage induced into a residual unusedwinding portion can be prevented from being raised no less than a supplyvoltage whereby the insulation of the winding needs not to bestrengthened.

[0015] In order to attain the object, the invention provides a windingswitching device of a three-phase AC motor comprising an AC motor inwhich a winding having each phase is formed by a plurality of windingsand a connecting terminal connecting the windings to each other and bothterminals of the winding having the phase are provided on an outside ofa motor, winding switching means for properly switching the connectingterminal, and

[0016] a variable frequency power source for supplying a variablevoltage having a variable frequency to the AC motor,

[0017] wherein the winding switching means includes a plurality ofthree-phase rectifying means connecting one of ends of the windinghaving the phase to the variable frequency power source and connectingthe other end and the connecting terminal to an input terminal on an ACside of the three-phase rectifying means for each phase, and asemiconductor switch provided to open and close both ends on a DC outputside of the three-phase rectifying means.

[0018] Moreover, the winding switching device of the three-phase ACmotor according to the first aspect of the invention is characterized inthat the three-phase rectifying means are formed into a three-phasefull-wave rectifying diode bridge.

[0019] Furthermore, the winding switching device of the three-phase ACmotor according to the first aspect of the invention is characterized inthat the DC output side of the three-phase rectifying means is connectedto a parallel circuit including a resistor and a capacitor through adiode provided in such a direction that a current flowing from thethree-phase rectifying means flows to the parallel circuit when thesemiconductor switch is OFF and does not flow backward from the parallelcircuit to the semiconductor switch when the semiconductor switch is ONat both ends on the DC output side of each of the three-phase rectifyingmeans.

[0020] In addition, the winding switching device of the three-phase ACmotor according to the first aspect of the invention is characterized inthat the DC output side of the three-phase rectifying means is connectedto a DC bus of the variable frequency power source through a diodeprovided in such a direction that a current flowing from the three-phaserectifying means flows to the DC bus of the variable frequency powersource when the semiconductor switch is OFF and does not flow backwardfrom the DC bus of the variable frequency power source to thesemiconductor switch when the semiconductor switch is ON at both ends onthe DC output side of each of the three-phase rectifying means.

[0021] Since the switching is carried out by the semiconductor, theswitching operation can be completed in a very short time with a smallnumber of semiconductor units.

[0022] Even if a mode for partially using the winding is selected,moreover, it is possible to avoid an extreme increase in a voltageinduced to the residual terminals.

[0023] In a winding switching device of a three-phase AC motorcomprising an AC motor in which a winding having each phase is formed bya plurality of windings and a connecting terminal connecting thewindings to each other and both terminals of the winding having thephase are provided on an outside of a motor, winding switching means forproperly switching the connecting terminal, and a variable frequencypower source for supplying a variable voltage having a variablefrequency to the AC motor, the winding switching means is constituted bya plurality of three-phase rectifying means connecting one of ends ofthe winding having the phase to the variable frequency power source andconnecting the other end and the connecting terminal to an inputterminal on an AC side of the three-phase rectifying means for eachphase, and a semiconductor switch provided to open and close both endson a DC output side of the three-phase rectifying means. Consequently,the following advantages can be obtained.

[0024] (1) A time required for switching a winding can be reduced.

[0025] (2) The number of semiconductor switch units for switching awinding can be reduced as much as possible to reduce a size and a costwithout using a switch having a mechanical movable section.

[0026] (3) Also in the case in which an intermediate point power of awinding is to be supplied, a voltage induced into a residual unusedwinding portion can be prevented from being raised to be equal to orhigher than a supply voltage and the insulation of the winding does notneed to be intensified.

[0027] (4) In the case of a discharging resistor being omitted to obtaina state shown in FIG. 5, moreover, an energy is not radiated as a heatloss by the resistor but is absorbed into the smoothing capacitor of thevariable frequency power source. Consequently, the energy can be reusedfor motor driving.

[0028] As a ripple effect, the winding can be switched in a considerablyshort time as compared with a switching method using a contact.Consequently, it is possible to minimize the influence of the switchingon a machine and a device to be loads.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029]FIG. 1 is a diagram showing the structure of a basic circuitaccording to a first embodiment of the invention.

[0030]FIG. 2 is a diagram showing a voltage state according to theinvention.

[0031]FIG. 3 is a diagram showing a switching sequence according to theinvention.

[0032]FIG. 4 is a diagram showing the structure of a circuit accordingto a second embodiment of the invention.

[0033]FIG. 5 is a diagram showing the structure of a circuit accordingto an applied variant of the first embodiment in accordance with theinvention (FIG. 1).

[0034]FIG. 6 is a diagram showing the structure of conventionalstar-delta winding switching.

[0035]FIG. 7 is a diagram showing the conventional art in which two setsof star windings are switched in series and parallel.

[0036]FIG. 8 is a diagram showing the structure of conventional windingswitching.

[0037]FIG. 9 is a diagram showing the conventional art in which two setsof inverters are combined.

[0038]FIG. 10 is a diagram showing the structure of the conventionalwinding switching.

BEST MODE FOR CARRYING OUT THE INVENTION

[0039] Embodiments of the invention will be described below withreference to the drawings. FIG. 1 is a diagram showing the structure ofa basic circuit according to a first embodiment of the invention. InFIG. 1, 1 denotes an inverter section to be a variable frequencyvariable voltage source for a 3-phase motor control which is constitutedby main circuit transistors Q1 to Q6. Terminals TP and TN are connectedto the DC output terminal of a converter. 2 denotes an AC motor and 12denotes a winding switching section. Each phase winding of the motor 2is formed by two coils, and intermediate terminals TU3, TV3 and TW3connected to the coils are taken as the external terminals of the motor.TU2, TV2 and TW2 to be the ends of winding terminals having each phasein the AC motor 2 are connected to output terminals TU1, TV1 and TW1having each phase in the inverter section 1, respectively.

[0040] The other ends TU4, TV4 and TW4 of the winding terminals havingeach phase in the AC motor 2 are connected to AC input terminals TU7,TV7 and TW7 of a three-phase diode bridge DB2 in the winding switchingsection 12, respectively. The intermediate terminals TU3, TV3 and TW3having each phase in the AC motor are connected to AC input terminalsTU6, TV6 and TW6 of a three-phase diode bridge DB1 in the windingswitching section 12, respectively. SW1 and SW2 connected across the DCoutput side to open and close the DC output sides of the three-phasediode bridges DB1 and DB2 are self-arc-extinguishing semiconductorswitching units such as a bipolar transistor and an IGBT.

[0041] The structure of the winding switching section 12 will bedescribed. D1 and D2 denote a diode connected to the DC output side ofthe three-phase diode bridge:DB1. D3 and D4 denote a diode connected tothe DC output side of the three-phase diode bridge DB2. The diodes D1and D2 serve to cause a current flowing in DB1 to a parallel circuit ofCR when the semiconductor switch SW1 is OFF and to prevent the currentfrom flowing backward from the parallel circuit of CR to SW1 when SW1 isON. The diodes D3 and D4 also serve to prevent the backward flow in thesame manner as the diodes D1 and D2. C denotes a capacitor and R denotesa discharging resistor. C and R are connected in parallel with eachother. One of ends on the cathode side of the diode D1 is connected toone of the ends of a CR parallel connecting terminal and one of ends onthe cathode side of the diode D3. One of ends on the anode side of thediode D1 is connected to the positive side terminal of the DC output ofthe three-phase diode bridge DB1 and the collector of SW1. One of endson the anode side of the diode D2 is connected to the other end of theCR parallel connecting terminal and one of ends on the anode side of thediode D4. One of ends on the cathode side of the diode D2 is connectedto the negative side terminal of the DC output of the three-phase diodebridge DB1 and the emitter of SW1. One of ends on the anode side of thediode D3 is connected to the positive side terminal of the DC output ofthe three-phase diode bridge DB2 and the collector of SW2. One of endson the cathode side of the diode D4 is connected to the negative sideterminal of the DC output of the three-phase diode bridge DB2 and theemitter of SW2.

[0042] Next, an operation in FIG. 1 will be described. When only SW1 isturned ON (SW2 is OFF), the motor terminals TU3, TV3 and TW3 areshort-circuited through DB1 so that a voltage is applied to a starconnection constituted by TU2-TU3, TV2-TV3, and TW2-TW3 to be a part ofmotor windings. Although a voltage is induced to the terminals TU4, TV4and TW4 by an electromagnetic coupling between the windings, theresistance value of the discharging resistor R is large. For thisreason, a current flowing to D3, R and D4 might be small enough to benegligible. With this structure, an impedance is lower than that in thecase in which all the motor windings are used. Therefore, this structurecan cause a sufficient current to flow even in a high frequency regionand is therefore suitable for a high-speed operation. On the other hand,when only SW2 is turned ON (SW1 is OFF), the motor terminals TU4, TV4and TW4 are short-circuited through DB2 so that a voltage is applied toa star connection constituted by all the windings TU2-TU4, TV2-TV4 andTW2-TW4. In this case, since the resistance value of the dischargingresistor R is great, a current flowing from the negative side terminalon the DC output side of DB1 to D1, R and D2 might be small enough to benegligible. With this structure, since an impedance is higher than thatin the case in which a part of the former motor windings is used, asufficient voltage can be applied also in a low frequency region and agreat torque can be generated for the same current. For this reason,this structure is suitable for an operation at a low speed. Accordingly,it is possible to enlarge a speed control range by selectively turningON SW1 or SW2 corresponding to an operating speed.

[0043]FIG. 5 shows the structure of a circuit according to an embodimentto be a variant of FIG. 1 according to the invention. The structure ofthe circuit in FIG. 5 is different from that of the circuit in FIG. 1 inthat the backward flow preventing diode of the winding switching sectionis connected to the parallel circuit of the capacitor C and the resistorR in FIG. 1, while the backward flow preventing diode is connected tothe DC bus of a variable frequency power source in FIG. 5. Morespecifically, the diodes of D1 and D3 are connected from a terminal TP1to the input terminal TP on the DC side of the inverter section 1 to bea variable frequency power source, and the anodes of D2 and D4 areconnected from a terminal TN1 to the input terminal TN on the DC side ofthe inverter section 1. As a result, the energy of a current flowingfrom DB1 and DB2 is not radiated as a heat loss by a resistor but isabsorbed into the smoothing capacitor of the variable frequency powersource and can be reused for the driving operation of a motor.

[0044]FIG. 2 represents, in a vector, the state of a voltage obtainedwhen SW1 is turned ON and SW2 is turned ON. It is apparent that only anequivalent voltage to a supply voltage is induced to the residualwinding terminals (TU3, TV4 and TW4) also when a high-speed winding(FIG. 2(a)) using a part of windings is selected.

[0045] Next, a winding switching method will be described. A sequencefor switching SW1 and SW2 has two methods as shown in FIG. 3. In FIG.3(a), first of all, a current is blocked on the inverter section 1 sidein response to a switching signal. In-the non-current state, switchingbetween SW1 and SW2 is carried out and a current is then caused to flowagain on the inverter section 2 side. A time t1 taken for blocking acurrent and then causing the current to flow again is required foractual switching. (SG1) indicates a switching signal for a winding whichis output from an inverter control circuit or an upper control devicefor controlling an inverter, (SG2) indicates a current flowing to amotor winding, and (SG3) and (SG4) indicate the conducting states of thesemiconductor switches SW1 and SW2 respectively. This method uses aconventional contactor and is carried out for prolonging the lifetime ofa contact. Also in the case in which the method is applied to theinvention, a unit is turned ON/OFF without a current so that anexcessive voltage can be prevented from being applied by switching.Since the operation of a semiconductor unit is carried out quickly, anoperating time can be shortened extraordinarily for the period t1 givingthe non-current as compared with the method using the contactor.

[0046] A switching method shown in FIG. 3(b) serves to carry outswitching without blocking a current in the inverter section 1. Althoughthe operation of the semiconductor is carried out very quickly, there isa possibility that a period for simultaneously turning ON SW1 and SW2might be generated due to a very short operation delay time. In order toeliminate the possibility, therefore, it is necessary to put a dead timet2 in which both of the semiconductor switches SW1 and SW2 are turnedOFF for a period in which they are turned ON respectively. Although itis sufficient that the dead time is very short by the high-speedswitching characteristic of the semiconductor (usually, severalmicroseconds or less), an energy (E=(½)Li²) generated by a current (i)stored in an inductance (L) of the motor winding is discharged for thisperiod. Consequently, an overvoltage is applied to a switching circuit.The first embodiment will be given. The capacitor C connected throughthe diodes D1, D2, D3 and D4 from both ends of SW1 and SW2 in FIG. 1serves to absorb the surge voltage, and R indicates a dischargingresistor. In FIG. 5 showing the variant of FIG. 1, SW1 and SW2 areconnected to the smoothing capacitor of the variable frequency powersource through the diodes D1, D2, D3 and D4. For this reason, thedischarging resistor is not required. In the case in which SW1 and SW2are to be switched in the non-current state shown in FIG. 3(a), it isnot necessary to always provide the capacitor C.

[0047]FIG. 4 shows a second embodiment of the invention. In theembodiment, a winding for each phase of a motor is divided into threeparts. The second embodiment is different from the first embodiment(FIG. 1) in that the number of divisions of the winding having eachphase of the motor is increased from 2 to 3 and a three-phase diodebridge DB3, diodes D5 and D6 and a semiconductor switch SW3 areadditionally provided corresponding to the increase in the number ofdivisions.

[0048] Next, description will be given to a structure in which a windingswitching section 13 according to the second embodiment is differentfrom the winding switching section 12 according to the first embodiment.One of ends on the cathode side of the diode D5 is connected to one ofends of a CR parallel connecting line terminal in the same manner asterminals on the cathode side of the diodes D1 and D3. One of ends onthe cathode side of the diode D5 is connected to the positive sideterminal of the DC output of the three-phase diode bridge DB3 and thecollector of SW3.

[0049] A terminal on the anode side of the diode D6 is connected to theother end of the CR parallel connecting terminal in the same manner asterminals on the anode side of the diodes D2 and D4. A terminal on thecathode side of the diode D6 is connected to a terminal on the negativeside of the DC output of the three-phase diode bridge DB3 and theemitter of SW3.

[0050] Also in the case of FIG. 4, moreover, a diode for preventing abackward flow can be connected to the DC bus of a variable frequencypower source in the same manner as in the structure of FIG. 5 to be thevariant of FIG. 1.

[0051] Since induction types, synchronizing types, rotating types ordirect-acting types are not distinguished for the AC motor used in theinvention, any AC motor can be applied.

[0052] Industrial Applicability

[0053] In a winding switching device of a three-phase AC motorcomprising an AC motor in which a winding having each phase is formed bya plurality of windings and a connecting terminal connecting thewindings to each other and both terminals of the winding having thephase are provided on an outside of a motor, winding switching means forproperly switching the connecting terminal, and a variable frequencypower source for supplying a variable voltage having a variablefrequency to the AC motor, the winding switching means is constituted bya plurality of three-phase rectifying means connecting one of ends ofthe winding having the phase to the variable frequency power source andconnecting the other end and the connecting terminal to an inputterminal on an AC side of the three-phase rectifying means for eachphase, and a semiconductor switch provided to open and close both endson a DC output side of the three-phase rectifying means. Consequently,the following advantages can be obtained.

[0054] (1) A time required for switching a winding can be shortened.

[0055] (2) The number of semiconductor switch units for switching awinding can be decreased as much as possible to reduce a size and a costwithout using a switch having a mechanical movable section.

[0056] (3) Also in the case in which an intermediate point power of awinding is to be supplied, a voltage induced into a residual unusedwinding portion can be prevented from being raised to be equal to orhigher than a supply voltage and the insulation of the winding does notneed to be intensified.

[0057] (4) In the case in which a discharging resistor is eliminated toobtain a state shown in FIG. 5, moreover, an energy is not radiated as aheat loss by the resistor but is absorbed into the smoothing capacitorof the variable frequency power source. Consequently, the energy can bereused for motor driving.

[0058] As a ripple effect, the winding can be switched in a considerablyshort time as compared with a switching method using a contact.Consequently, it is possible to minimize the influence of the switchingon a machine and a device to be loads.

1. A winding switching device of a three-phase AC motor comprising an ACmotor in which a winding having each phase is formed by a plurality ofwindings and a connecting terminal connecting the windings to each otherand both terminals of the winding having the phase are provided on anoutside of a motor, winding switching means for properly switching theconnecting terminal, and a variable frequency power source for supplyinga variable voltage having a variable frequency to the AC motor, whereinthe winding switching means includes a plurality of three-phaserectifying means connecting one of ends of the winding having the phaseto the variable frequency power source and connecting the other end andthe connecting terminal to an input terminal on an AC side of thethree-phase rectifying means for each phase, and a semiconductor switchprovided to open and close both ends on a DC output side of thethree-phase rectifying means.
 2. The winding switching device of thethree-phase AC motor according to claim 1, wherein the three-phaserectifying means are formed into a three-phase full-wave rectifyingdiode bridge.
 3. The winding switching device of the three-phase ACmotor according to claim 1, wherein the DC output side of thethree-phase rectifying means is connected to a parallel circuitincluding a resistor and a capacitor through a diode provided in such adirection that a current flowing from the three-phase rectifying meansflows to the parallel circuit when the semiconductor switch is OFF anddoes not flow backward from the parallel circuit to the semiconductorswitch when the semiconductor switch is ON at both ends on the DC outputside of each of the three-phase rectifying means.
 4. The windingswitching device of the three-phase AC motor according to claim 1,wherein the DC output side of the three-phase rectifying means isconnected to a DC bus of the variable frequency power source through adiode provided in such a direction that a current flowing from thethree-phase rectifying means flows to the DC bus of the variablefrequency power source when the semiconductor switch is OFF and does notflow backward from the DC bus of the variable frequency power source tothe semiconductor switch when the semiconductor switch is ON at bothends on the DC output side of each of the three-phase rectifying means.